1. Field of the Invention
The present invention generally relates to the conversion of chemical energy to electrical energy. More particularly, this invention relates to the preparation of a new cathode active material for nonaqueous electrolyte lithium electrochemical cells. The cathode material comprises a highly crystalline mixed metal oxide, such as ε-phase silver vanadium oxide (SVO, Ag2V4O11), prepared using a two-step synthesis process.
In an exemplary synthesis, silver nitrate (AgNO3) is first reacted with vanadium oxide (V2O5) in a decomposition reaction under an inert atmosphere, resulting in an SVO intermediate having a reduced stoichiometric oxygen content. The SVO intermediate is then subjected to a separate oxidation reaction to produce the highly crystalline product SVO. The synthesis protocol of the decomposition reaction followed by the oxidation reaction produces a cathode active material with increased crystallinity and increased surface area, which cannot be obtained by prior art methods.
2. Prior Art
U.S. Pat. Nos. 4,310,609 and 4,391,729, both to Liang et al., disclose the preparation of silver vanadium oxide by a thermal decomposition reaction of silver nitrate with vanadium oxide conducted under an air atmosphere. This decomposition reaction is further detailed in the publication: Leising, R. A.; Takeuchi, E. S. Chem. Mater. 1993, 5, 738-742, where the decomposition synthesis of SVO from silver nitrate and vanadium oxide under an air atmosphere is presented as a function of temperature.
In another reference: Leising, R. A.; Takeuchi, E. S. Chem. Mater. 1994, 6, 489-495, the decomposition synthesis of SVO from different silver precursor materials (silver nitrate, silver nitrite, silver oxide, silver vanadate, and silver carbonate) is described. In this publication, the heating temperature is held constant at about 500° C. and the synthesis occurs under an air or argon atmosphere.
Interestingly, SVO materials prepared in a decomposition reaction under an argon atmosphere have very different thermal properties, x-ray diffraction patterns, and electrochemical discharge properties than SVO samples similarly prepared under air. In the latter Leising et al. publication, the product of the decomposition reaction under argon is a mixture of phases of SVO materials. Multi-phase SVO is not the same as a highly crystalline ε-phase Ag2V4O11.
In addition, the electrochemical discharge properties of lithium test cells demonstrate that multi-phase SVO prepared under an argon atmosphere has significantly less useable capacity than SVO prepared under an oxidizing atmosphere. Thus, the prior art teaches away from the use of an inert gas, such as argon, in the synthesis protocol of a mixed metal oxide, such as a highly crystalline SVO.
In comparison to SVO prepared by a decomposition reaction, U.S. Pat. No. 5,221,453 to Crespi discloses the synthesis of silver vanadium oxide by a chemical addition reaction of silver oxide and vanadium oxide starting materials. Crespi's chemical addition or combination reaction is described as a single step synthesis where the heating takes place in an inert atmosphere or separately in an oxidizing atmosphere. No combination of atmospheres, or combination of heating steps, is disclosed.
Also, the preparation of SVO from silver oxide and vanadium oxide is well documented in the literature. In the publications: Fleury, P.; Kohlmuller, R. C. R. Acad. Sci. Paris 1966, 262C, 475-477, and Casalot, A.; Pouchard, M. Bull Soc. Chim. Fr. 1967, 3817-3820, the reaction of silver oxide with vanadium oxide is described. Wenda, E. J. Thermal Anal. 1985, 30, 879-887, presents a phase diagram of the Ag2O-V2O5 system in which the starting materials are heated under oxygen to form SVO and other silver vanadium oxide materials. It is significant to note that Fleury and Kohlmuller commented that the thermal analysis of the Ag2O-V2O5 system should be conducted under flowing oxygen to avoid the in-situ formation of bronzes. Bronzes of silver vanadium oxide are referred to as AgxV2O5 type materials, which include significant amounts of unreacted vanadium(IV). An ε-phase SVO (Ag2V4O11) is not a bronze, since it only contains vanadium(V). Thus, Fleury and Kohlmuller were warning that the heat treatment of SVO starting materials under a non-oxidizing atmosphere (such as argon) results in the formation of reduced stoichiometric silver vanadium oxide products.
U.S. Pat. No. 5,955,218 to Crespi et al. discloses the process of heat-treating thermal decomposition produced SVO to improve the material's electrochemical performance. In this patent, SVO is first synthesized in a decomposition reaction under air, as described in U.S. Pat. Nos. 4,310,609 and 4,391,729, both to Liang et al., however, at a somewhat lower temperature of 360° C. The '218 patent to Crespi et al. demonstrates that adding a second oxidation heat treatment to the Liang et al. decomposition reaction product increases the crystallinity of the resulting SVO material. However, the Crespi et al. SVO material of increased crystallinity still does not have the highly crystalline characteristics of the SVO material of the present invention.